Nano-optical reporters of dynamic mechanotransduction in the immune system Abstract Cellular signaling and health are governed in large part by mechanical forces. We experience these mechanical forces every time our heart beats, our ears hear, or a wound heals; mechanical forces also influence cell differentiation, tissue organization, and diseases such as cancer and heart disease. Among cells, leukocytes are some of the most physically active, able to quickly move through tight interstitial interfaces, reorganize their cytoskeleton within minutes, and form dynamic, force-inducing interfaces (immune synapses) with pathogenic cells that target them for death. Yet, despite its importance in immunology, the influence of local force at the leukocyte- pathogen cell interface remains poorly understood. In particular, the few tools that exist for measuring inter-cellular forces today are either unsuitable for use in living cells and tissues or do not have a sufficiently-high stability and dynamic range to accurately record forces throughout the lifetime of the immune synapse. Here, we unite nanophotonics and fundamental immunology to develop a new class of in vivo force probes to monitor dynamic forces at the immune cell/pathogen interface with deeply sub-cellular spatial resolution. Our design is based on inorganic nanoparticles that, when excited in the near-infrared, emit light of a different color (wavelength) and intensity in response to tensile or compressive strain. The nanoparticles are small (<30nm), bio-compatible, do not bleach or photoblink, and have sharp emission linewidths. We will use these color- changing sensors to record local, dynamic forces exerted by innate and adaptive immune system cells, including those occurring during macrophage phagocytosis and at cytotoxic T-cell immune synapses. Our proposed work aims to unravel the role of force inhomogeneities in regulating local chemical release at immune synapses; the role of force in lymphocyte differentiation; and the role of force in evasion of immune attack. Addressing such questions could not only uncover important mechanotransduction pathways, but also aid in development of new immunological treatments and vaccines.